Biomass gasification is a complex thermo chemical process that consists of a number of elementary chemical reactions, beginning with the partial oxidation of a lignocellulosic fuel with a gasifying agent, usually air, oxygen or steam.Biomass+O2 (or H2O)→CO+CO2+H2O+H2+CH4 +other gaseous hydrocarbons+Tar+Ash
In a gasification process, carbonaceous material undergoes various chemical reactions at various temperature ranges, i.e. from ambient to 1000° C. The nature of reactions is given below:    At 400-600° C.: The carbonaceous material decomposes into char and volatile matter.    At 600-1000: Carbon reacts with oxygen to produce carbon dioxide and heat and subsequently the CO2 thus produced reacts with C (char) to produce CO.    At 1000° C.: The steam is used to regulate the temperature and the steam used reacts with carbon to produce carbon mono-oxide and hydrogen.
The tar is formed at 400-600° C., wherein the volatile matter undergoes secondary decomposition and polymerization to produce tar. Tars are problematic in integrated biomass gasification systems for a number of reasons. Tars can condense in exit pipes and on particulate filters leading to blockages and clogged filters and other complications. As mentioned previously, existing process of gasification utilizes very complex and expensive purification system to overcome the tar problem.
Several technology such as mechanism methods (scrubbers, filters, cyclone, ESP and RSP), self-modification methods (selecting better gasifier and optimizing operation parameters) and newly developed plasma technology have been tested with certain degree of success.
Mechanism methods only remove or capture the tar from product gases, while the energy in tar is lost. The self-modification can reduce the tar and convert the tar into useful gases; however, modification of operation parameter is at the expense of reduction in the heat value of gases. A new two-stage gasifier can produce the syngas with low tar content and high heat value.
Thermal cracking and catalytic cracking are used to decompose or reduce tar though there are still some disadvantages. In order to gets highly efficient tar decomposition, the temperature of thermal cracking needs to be very high, which can result in higher operating cost. Catalyst cracking can modify the composition of product gases at very low temperature.
For catalyst cracking shortcomings exists such as the commercial Ni-based and alkali metal catalysts will be inactive by deposited carbon and H2S, as for dolomite catalyst, broken particles also decreases the activity. Plasma technology cannot only effectively remove fly ash, NOx and SO2, but also sharply decreases the formation of tar during biomass gasification. A novel catalytic deposition (intimate contact between biomass and the catalyst) inside the biomass can overcome the disadvantages.
US patent 20070000177A1 describes the process of low temperature catalytic gasification of carbonaceous material achieves high carbon conversion to methane at mild temperature for fuel application.
U.S. Pat. No. 7,132,183 describes gasification process of carbonaceous feed stock in a gasifier with high temperature operating in the range of 700°-1600° C. and a portion of gases produced there from electrochemically oxidized in a fuel cell.
WO20070044342 patent describes the function of catalytic function of reforming a tar contained in the gas produced by the gasification reaction which absorbs CO2 contained in a gas by a chemical to acceleration due to thermal transfer of flowable heat carrier and a chemical.
WO2006031011 provides a gasification technique includes first stage fluidized bed catalytic gasification and second stage gasification of tar and catalytic reforming to convert nitrogen in tar and HCN in a flammable gas into NH3. The temperature of a total process is lower than the melting point of ash; powdery ash is generated and thus easily treated.
There have been extensive researches for evolving a tar free gasification process for conversion of coal or biomass into synthesis gas. The crux of tar generation process lies in reaction mechanisms involved leading to tar formation.
Till date all technologies involve tar elimination by removing the tar outside the gasifier after the gas is produced. However, no technology is available to prevent tar formation at pyrolysis step during gasification.